The present invention relates to dispersible emulsion polymer systems which are functional as pressure-sensitive adhesives. The adhesives of the instant invention provide an ecologically friendly replacement for pressure sensitive adhesives which hinder clean paper recovery during repulping.
For a paper to be repulpable, the adhesive in a label or tape construction must be water (alkali) dispersible or water soluble and must pass TAPPI Useful Method 213 "Repulpability of Splice Adhesive Compositions," incorporated herein by reference. Most rubber-based and acrylic adhesive systems are not repulpable. During the repulping process (fiber shearing), the adhesives tend to agglomerate and form globules, commonly referred to in the trade as "stickies." Stickies adversely affect paper quality and have a negative impact on the process of recycling paper.
Several approaches can be taken to remove adhesive stickies. One has been to use water-dispersible adhesive microsphere technology prepared by processes described in U.S. Pat. Nos. 3,691,140 to Silver, 4,155,152 to Baker et al., 4,495,318 and 4,598,212 to Howard, and 4,810,763 to Mallya et al., each incorporated herein by reference, and applied at appropriately low coatweights. While the adhesives disperse in the paper fiber adequately during the repulping process, the adhesive is not removed from the paper fiber. Rather, the adhesive microspheres accumulate in the paper fiber, reducing paper quality and limiting the number of times the paper fiber can be repulped or recycled.
Water-soluble adhesive systems which provide for complete adhesive removal from the fiber have also been proposed. Typically, commercial water-soluble products have suffered from (a) poor shelf life (the tape or label is sold in a sealed plastic bag); (b) poor humidity and/or heat-aged performance; and (c) a propensity to bleed into the paper facestock (which reduces peel performance). Each of these problems is related to the high water-sensitivity of the respective products. Such water dispersible adhesives and tapes are described, for instance, in U.S. Pat. Nos. 3,441,430 to Peterson, 3,865,770 to Blake, 4,052,368 to Larson, 4,413,080 to Blake, and 4,569,960 also to Blake, each incorporated herein by reference.
There is a need therefore to provide adhesives for a general purpose label and tape construction which are repulpable and not sensitive to changes in relative humidity and where no special precautions are required for storage or use as an adhesive for a label or tape.